This invention relates to methods and arrangements for achieving synchronization of network elements in a telecommunications synchronous network.
Synchronous transport techniques are becoming widely used in the telecommunications field. The standard protocol for synchronous transport is commonly referred to as the synchronous digital hierarchy (SDH) or, in North America, as SONET. The traffic in such networks is transported in frames which are multiplexed up into virtual containers at various levels of multiplexing. A key requirement in a synchronous system is that each network element receives a timing or clock reference whereby it can adjust its local timing to ensure synchronization. Synchronous networks have the significant advantage of simplifying the process of switching and of demultiplexing channels and thus avoid the need for the so-called multiplexer/demultiplexer mountains that were associated with previous plesiochronous techniques.
Typically such a network embodies a backbone ring topology comprising a number of interconnected rings. This topology is widely used as it facilitates the provision of built-in protection paths for the network elements or nodes. Traffic can pass in both directions around a ring. In the event of failure of part of a ring, the nodes or elements in that ring can still receive and transmit over the remaining part of the ring.
A particular problem that is inherent in such a ring structure is that of preventing the establishment of synchronization loops either on system set up or during recovery from a fault such as a fibre break. In a typical synchronous network, timing of a network element is derived from one of a number of highly accurate reference sources, e.g. atomic clocks, that are distributed around the network.
In paractice, a network element will receive the synchronization signal either directly from such a source or relayed from an adjacent element that is already synchronized to the source. For protection purposes it is necessary that a network element can synchronize from an alternative source, in the the event of a failure of a ring fibre path, to that which that element is currently synchronized. This facility to synchronize from an alternative source has however been found difficult to timplement in a ring network. In a ring based network where each network element or node can access two or more substantially identical synchronization sources, there is risk of establishing a closed synchronization loop, i.e. positive feedback. Under such conditions, the network element may erroneously attempt to synchronize with itself around the ring with a consequent loss of timing accuracy. This is a particular problem following failure of one synchronization with an alternative source. Under such conditions, a ring element or node has no means of determining whether a received signal to which it is attempting to synchronize has originated from an active synchronization source, or whether that signal has in fact originated from the node itself and has been relayed around the ring. To reduce or prevent the risk of such an occurrence, the network desginer will normally deliberately limit the number of allowed synchronizations paths in a ring. However, this in turn reduces the robustness of the ring to other system faluts.
A further problem is that of carrying the synchronization signal between adjacent rings of the network. In order to provide the required protection, it is necessary to provide two paths between the rings. These paths carry identical signals, including synchronization signals, but only one path will be selected as the traffic and signal carrying path. If that path fails, causing the second path to be selected, the nodes within the ring being served will automatically attempt to synchronize to the signal carried on the second path. However, a node in the ring will have no way of telling whether a received synchronization signal has arrived via the second path or whether that signal has originated from the node itself and been transmitted around the ring. Currently this problem is addressed by appropriate design of xe2x80x98subsidiaryxe2x80x99 ring nodes, but this is inconvenient as it then requires different products to be manufactured and supplied.
An object of the invention is to minimize or to overcome the above disadvantages.
A further object of the invention is to provide an improved arrangement and method for synchronization of a network element in a synchronous telecommunications network.
According to a first aspect of the invention, there is provided a synchronous digital communications network comprising a plurality of network elements interconnected by communications paths, and having a plurality of synchronization timing reference sources for providing timing signals to which said network elements can synchronize, and wherein each said synchronization source is allocated a respective unique identity whereby each said network element is informed of the identity of the derivation of the timing reference signal to which that element is synchronized.
According to a further aspect of the invention, there is provided a method of synchronizing a synchronous digital communications network comprising a plurality of network elements interconnected by communications paths, and having a plurality of synchronization timing reference sources for providing timing signals to which said network elements can synchronize, the method comprising allocating to each said synchronization source a respective unique identity, and transmitting said timing signals each together with the respective unique identifier to the network elements whereby each said network element is informed of the identity of the timing reference source to which that element is synchronized.
According to a further aspect of the invention, there is provided a method of synchronizing a synchronous network element to a synchronization signal in a telecommunications network, the method including providing to that network element an indication of the origin of the synchronization signal so as to avoid use by the network element of a synchronization signal originating from itself.
According to another aspect of the invention, there is provided a method of synchronizing a first synchronous network ring to a second synchronous network ring via first and second paths via which paths said rings are coupled, the method comprising, at the first ring, transmitting an identical timing reference signal together with an identifier for that timing signal on each said path from the first ring to the second ring, and, at the second ring, receiving said timing signals from the first and second paths, providing said received timing signals with respective first and second new identifiers, and selecting either one or the other of said newly identified timing signals for synchronization of the second ring.
According to another aspect of the invention, there is provided a synchronous digital communications network incorporating a plurality of network elements or nodes interconnected by communications paths, said network having a primary synchronization timing reference source to which all network nodes are synchronized, and a secondary, stand-by synchronization source with which said elements or nodes can synchronize in the event of failure of the primary reference source, and wherein said reference sources are each provided with a respective identifier whereby each said network element or node is informed of the identity of the timing reference source to which that element or node is synchronized.
According to a further aspect of the invention, there is provided a method of synchronizing a network element in a synchronous digital communications network incorporating a plurality of network elements disposed in path protected rings and having two or more timing reference sources, the method comprising; allocating a unique identifier comprising an artificial ranking to said reference sources such that one source is identified by a highest artificial ranking as a primary reference source, the other source or sources being respectively lower ranked as stand-by reference sources, synchronizing each said element to the primary reference source, and, in the event of a failure of the primary reference source, inhibiting further use of that source by each said network element and synchronizing each said network element with the next available highest ranked stand-by source.
According to another aspect of the invention, there is provided a synchronous digital communications network incorporating a plurality of network elements disposed in path protected rings and having two or more timing reference sources, said reference sources each being allocated a ranking such that one source is highest ranked as a primary reference source, the other source or sources being respectively lower ranked as stand-by reference sources, synchronizing means disposed at each said element for effecting synchronization of that element to the primary reference source, and control means responsive to a failure of the primary reference source for barring further use of that source by each said network element and for instructing each said network element to synchronize with the highest ranked stand-by source.
According to a further aspect of the invention, there is provided a network node or element for use in a synchronous digital communications network incorporating a plurality of network elements disposed in path protected rings and having two or more timing reference sources, said reference sources each being allocated a ranking such that one source is highest ranked as a primary reference source, the other source or sources being respectively lower ranked as stand-by reference sources, wherein the node the incorporates; a local oscillator, means for receiving preferentially a synchronization signal from said primary reference source so as to synchronize the local oscillator with that synchronization signal, output means for forwarding the synchronization signal to an adjacent node, and means responsive to a failure of the primary reference source for barring further use of that source by said network node and for synchronizing said network node oscillator with the highest ranked stand-by source.
The method permits a network element to be aware that a synchronization signal with which it could synchronize has in fact originated from the network itself and should therefore not be used thus preventing the establishment of a synchronization loop.
Conveniently, the synchronization reference sources are given each a unique ranked quality level (QL) by the operator via the network manager, the primary source being ranked at the highest level (QL 1), the one or more secondary or stand-by sources being ranked at respective lower levels. This quality level can be transmitted in the section overhead (SOH) to provide to a node receiving the synchronization signal a unique identifier of the source from which that timing reference signal has originated. In a further embodiment, the quality level message can be modified to indicate to a receiving node the path whereby that message has reached the node, e.g. so as to differentiate between a pair of paths in a protection arrangement.
It will be understood that the techniques described herein are applicable both to the North American SONET (ANSI) standard and to the European SDH (ITU) standard.